Method and apparatus for measuring the equivalent series resistances of a capacitor

Abstract

An instrument for measuring the equivalent series resistance of a capacitor includes a plurality of constant current generators across which a capacitor under test is adapted to be connected. A selected one of the generators is periodically enabled to supply a constant current to the capacitor and the voltage across the capacitor is measured and displayed a short time after the generator is enabled, after which time the capacitor is discharged and the measuring cycle is repeated.

What is claimed: 
     
       1. Apparatus for measuring and displaying the equivalent series resistance of a capacitor comprising in combination current generator means for applying to said capacitor a constant charging current for a first predetermined period of time to cause the voltage across said capacitor to increase initially to a first level which is proportional to said equivalent series resistance of said capacitor and thereafter to increase at a rate proportional to said charging current,   timer means for timing out a second period beginning with the initial application of said charging current to said capacitor and ending when said voltage across said capacitor is substantially at said first level,   voltage measuring means responsive to said timer means for measuring said voltage across said capacitor at the end of said second period, and   means responsive to the value of said voltage measured by said voltage measuring means for displaying a visual representation of the equivalent series resistance of said capacitor.   
     
     
       2. Apparatus according to claim 1, further comprising means responsive to said timer means for discharging said capacitor for a third predetermined time period beginning after the end of said first period.   
     
     
       3. Apparatus according to claim 2 wherein said timer means comprises a first pulse generating means for controlling the period during which said current generating means applies charging current to said capacitor and the period during which said capacitor is discharged.   
     
     
       4. Apparatus according to claim 3 wherein said timer means comprises a second pulse generating means for generating a pulse having its leading and trailing edges respectively coinciding with the beginning and end of said second period.   
     
     
       5. Apparatus according to claim 1 wherein said second period has a duration of no more than one microsecond.   
     
     
       6. Apparatus according to claim 5 wherein said current generator means comprises a plurality of constant current generators respectively producing different output currents, and   means for selecting one of said plurality of current generators for charging said capacitor.   
     
     
       7. A method of measuring the equivalent series resistance of a capacitor, comprising in combination the steps of discharging said capacitor and then connecting it across a source of constant current to charge said capacitor and to develop across said capacitor a voltage which initially increases substantially instantaneously to a first voltage level and which thereafter increases linearly at a rate proportional to the capacitance of said capacitor and to the value of said current,   measuring said first voltage level across said capacitor, and   developing a signal proportional to said first voltage level divided by a signal proportional to said current to provide a signal which is proportional to the equivalent series resistance of said capacitor.   
     
     
       8. A method according to claim 7, wherein said step of measuring is carried out by measuring said first voltage at a time no later than one microsecond after connecting said capacitor to said source of current.   
     
     
       9. A method according to claim 8, wherein said step of measuring is carried out by measuring said first voltage at a time no later than 0.5 microsecond after connecting said capacitor to said source of current.